Use of bioactive glass

ABSTRACT

The present invention relates to the use of a bioactive glass as an air abrasive agent for use in the treatment of dental disorders.

The present invention relates to the use of bioactive glasses asabrasive agents in the management of dental pain.

Dental pain is a common problem affecting 17% of the population at anyone time and increases in frequency with age. (Litkowski L J., Hack GD., Sheaffer H B., Greenspan D C., 1997, Occlusion of dentine tubules by45S5 Bioglass®, Bioceramics 10 Procs 10^(th) Int., Symposium on ceramicsin Medicine pp 411-414, Ed. Sedel L & Rey C. Elsevier Science Ltd). Theprinciple causes of this dental discomfort arise from both abrasion andacid erosion of external root dentine, which has either been revealedafter gingival recession (due to periodontal disease or as a feature ofmaintaining teeth further into old age) or bas developed due to dentalcaries i.e. the acid dissolution of dental tissues resulting frombacterial plaque metabolic activity.

One of the currently accepted causes of dental pain is the hydrodynamictheory (Litkowski, et al 1997) whereby movement of fluid through thedentine tubule tracts excites either the odontoblasts, whose processes(once) occupied the tubules, or adjacent nerve fibres directly. In thenormal healthy state, these tubules are closed off by the overlyingenamel crown or cementum of the root. When dentine is exposed either bythe ravages of periodontal disease (gingival recession) or by dentaltreatment eg acid etching after cutting a restoration cavity, thetubules are lain open, allowing large fluid movements and consequentialdental pain.

Regardless of its similarities, toothache resulting from cavityformation relates to a different problem area to hypersensitive dentine.Sensitivity associated with caries, and pain caused by irritation isusually treated by removal of decay and restoration by filling. At thebottom of the prepared cavity, a commercially available preparation isplaced against the pulp, the biologically active component of suchpreparation is usually calcium hydroxide. At the cell level, thestrongly alkaline calcium hydroxide first induces irritation, whichleads to the necrotisation of the tissue. Over a longer time span,however, it promotes the healing process. The result of the treatment isthe formation of reparative secondary dentine. The formed tissue layerseparates the pulp from the damaged area or the filling, but its effecton the mineralization of dentine tubules is minimal.

During filling, the dentinal tubules can also be closed by glass ionomercement, or with different preparations based on polymer chemistry(binder plastics, resins, and dentine adhesives). These substances closedentinal tubules mechanically and improve the retention of the fillingbeing prepared.

The epidemiological data describing the extent of the problem caused byhypersensitive dentine and the need for its treatment is limited.However, it is widely accepted that tubule occlusion by varnishes,resins or crystal precipitation will reduce or eliminate dentinesensitivity (Litkowski, et al 1997). The duration of the relief equatesto the service lifetime of the occluding material (Litkowski, et al1997), which can be all too brief e.g. if applied to a root surfacecontinually abraded by a toothbrush.

Recently, in connection with tooth hypersensitivity, Litkowski, et al1997 has shown in vitro that bioactive glasses can occlude exposedtubules and encourage re-mineralisation of the tooth surface.

U.S. Pat. No. 5,891,233 discloses preparations containing bioactiveglass which act to induce mineralisation in exposed dentine and theiruse in the treatment of pulpal irritation i.e. tooth hypersensitivityand/or tooth strengthening. The bioactive glass demonstrated must beapplied and maintained in moist form to encourage chemical interactionbetween the glass phase and the dentine.

Thus, in U.S. Pat. No. 5,891,233 the bioactive glass preparations arepresented in the form of solutions, suspensions and pastes. In use, thebioactive glass preparation is placed in direct contact with the area ofthe tooth to be treated. For example, the paste or solution is placed ina periodontal pocket, in a drilled cavity or spread onto a polishedsurface or otherwise exposed dentinal surface. The bioactive preparationis then covered with protective packing or cementum to preventdisplacement of the preparation.

However such methods suffer from the disadvantage that the area to betreated must first be prepared using conventional dental techniques. Forexample, in the case of cavity formation, the caries must first beremoved with a drill or the like before the bioactive paste can beapplied. Moreover, as mentioned above, when applied as a paste thebioactive preparation must be retained in place with protective packingfor an extended period. In use such packing is prone to becomingdetached and the paste then simply washes away. Moreover, when used totreat hypersensitivity the packing is often visible during the period oftreatment. Such unsightly packing can lead to premature removal of thepacking by the patient and thus failure of the treatment.

U.S. Pat. No. 5,735,942 discloses a novel silica based bioactive glasscomposition having a particle size range <90 μm for use in conjunctionwith a delivery agent such as a toothpaste, and the use of suchcompositions in treating dentine hypersensitivity.

U.S. Pat. No. 6,086,374 reports that the compositions of U.S. Pat. No.5,735,942 may be used to remineralise enamel and prevent tooth decay.

Air abrasion as a means of cutting or preparing tooth substrate surfacesby harnessing the transferred kinetic energy of alumina particlesaccelerated in a controlled compressed gas stream has been known sincethe 1950s. The abrasive stream cuts (abrades) through the targetsubstrate by repeated localised impacts serially removing material fromthe point of aim. More recently, dental “air polishing” employingbicarbonate of soda as an abrasive for tartar removal has gainedacceptance.

The use of other gases as a propellant (eg CO₂ or N₂) is included in thedefinition of “air abrasion” and the use of water or other fluids to actas dust suppression agents (regardless of potential contribution to theoverall cutting effect) are also included, however delivered—eitherincluded in the gas stream or entrained around it (e.g. The Aquacut airabrasive machine—Medivance Instruments Ltd, Harlesden, London).

We have now found that by using bioactive glass as an abrasive agent(cutting and/or surface peening agent) in a conventional air abrasionsystem, benefits are observed in the cutting of both tooth enamel anddentine and in the delivery of the bioactive glass.

Accordingly the present invention provides a method of treatment forand/or prophylaxis of a person suffering from or susceptible to dentalhard tissue and pulpal disorders, defined herein to include dentalcaries, pain, tooth wear, discoluration, dentine hyper-sensitivity anddental tissue congenital malformations, which method comprisescontacting the affected area with bioactive glass using an air abrasionsystem.

Alternatively the present invention provides the use of a bioactiveglass in the manufacture of an air abrasive agent for use in thetreatment of dental disorders.

Thus the present invention is based upon the observation that whenapplied through a conventional air abrasion system the bioactive glassparticles and fragments thereof become embedded in the surface of thetreated area providing long term effect and minimising the amount ofglass lost by erosion. The embedded bioactive glass provides long termeffect, encouraging rapid re-mineralisation of the affected area,accelerating surface healing and reducing the patient's dental pain.

The fact that particles of bioactive glass are actually embedded in thesurface of the treated area obviates the need for protective packing toprevent their displacement, thereby reducing the risk that thepreparation will be washed away and increasing the success rate of thetreatment.

Moreover, bioactive glass may be used as an abrasive agent in the airabrasive system to cut and abrade enamel and cariously damaged surfaces(i.e. de-mineralised enamel & dentine). Therefore the present inventionobviates the need for a separate preparation step as required when usingbioactive glass pastes and solutions to treat dental pain associatedwith caries.

Further advantages arise by carefully controlling the hardness and/orshape of the bioactive glass to be used, different types of dentalmaterial may be cut and/or abraded. Thereby giving rise to differentialcutting and minimising the possibility of cutting too far.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 compares untreated carious dentine (U) with carious root dentinethat has been air abraded with 45S5 bioactive glass (A) viewed with ascanning electron microscope (SEM).

FIG. 2 shows the enamel (E)—dentine (D) junction (EDJ) of a prepareddental surface that has been air abraded with 45S5 bioactive glassparticles viewed with a SEM.

FIG. 3 illustrates SEM examination of the dental surface shown in FIG. 2having been cleaved along an axis perpendicular to that of the treatedsurface (D).

FIG. 4 illustrates a section of human dentine, mounted in an orthodonticretainer style baseplate. The Figure compares bioactive glass treatedsurface (B) and alumina treated area (A). Intervening dentine bridge (D)provides a negative control for the surfaces.

FIG. 5 shows comparable Tandem Scanning Confocal surface reflectionimages at ×400 magnification of the experimental dentine surfaces. (A),(B) and (D) in—FIG. 4—taken over a full week wear period. Images A and Dof FIG. 5 illustrate area (B—FIG. 4) at the beginning and end of thetrial period respectively. Images B and E of FIG. 5 illustrate area(D—FIG. 4) at the beginning and end of the trial period respectively.Images C and F of FIG. 5 illustrate area (A of FIG. 4) at the beginningand end of the trial period respectively.

FIG. 6 shows two tynes of a 58S sol-gel bioactive glass comb one abradedwith 45S5 bioactive glass and the other with alumina, viewed under SEM.

FIG. 7 shows tandem scanning confocal fluorescence microscopy images ofthe tooth specimens retrieved from the experiment in FIG. 5, having beensectioned

FIGS. 8 a and 8 b show the effect of network modifiers (hardening andsoftening agents) and density on glass hardness.

The term “bioactive glass” as used herein refers to a glass or ceramicmaterial comprising Si-oxide or Si-hydroxide which is capable ofdeveloping a surface calcium phosphate/hydroxy-carbonate apatite layerin the presence of an aqueous medium, or at the interface of bodytissues and the glass, so producing a biologically useful response.

Bioactive glasses suitable for use with the present invention includethe silicon based bioactive glasses derived from the Sol-Gel process(Hench L L., West J K., 1990, The Sol-gel Process, Chem. Reviews, 90,33-72) or the Melt process (Hench L L., Wilson J., 1993 Introduction toBioceramics. Publisher: World Scientific).

Although it may be possible for a bioactive glass lacking a source ofcalcium or phosphorus to generate an apatite layer in vivo by utilisingendogenous sources of these ions, typically a bioactive glass willcomprise a source of at least one of calcium or phosphorous in additionto a source of Si-oxide or Si-hydroxide. Typically the bioactive glasswill comprise a source of calcium. Optionally the bioactive glass maycontain further hardening and/or softening agents. Such softening agentsmay be selected from: sodium, potassium, calcium, magnesium, boron,titanium, aluminum, nitrogen, phosphorous and fluoride. Additions ofsodium, potassium, calcium and phosphorus are most commonly used, toreduce the melting temperature of the glass and to disrupt the Sinetworks within it. Optionally, hardening agents such as TiO₂ may beincluded in the glass composition. Its presence would allowcrystallization to occur within its structure, so producing aglass-ceramic material, whose hardness will be greater than that of theglass alone. This will be of most benefit in producing a bioactiveabrasive for cutting the harder dental structures e.g. enamel asdiscussed below.

Thus, composition ranges for bioactive glasses which may be used withthe present invention are as follow:

SiO₂ or Si(OH)₂  1-100% CaO 0-60% P₂O₅ 0-60% Na₂O 0-45% K₂O 0-45% MgO0-40%

Plus additions of Na, K, Ca, Mg, B, Ti, Al, P, N and F as necessary.

Preferably, a bioactive glass will contain between 30 and 100% Si-oxideor Si-hydroxide, more preferably between 40 and 85%.

In a further preferred embodiment the bioactive glass will containbetween 5 and 60% Ca, more preferably between 30 and 55%.

With respect to a source of phosphorus, the bioactive glass will containbetween 5 and 40% P, more preferably between 10 and 30%.

Thus, in one embodiment the bioactive glass will comprise SiO₂, CaO andP₂O₅. Preferably the bioactive glass includes from 44 to 86 weight %SiO₂, from 4 to 46 weight % CaO and from 3 to 15 weight % P₂O₅.Preferably the bioactive glass is prepared by the sol gel route andcomprises from 55 to 86 weight % SiO₂, from 4 to 33 weight % CaO andfrom 3 to 15 weight % P₂O₅. Preferably such a bioactive glass has thecomposition 58 weight % SiO₂, 33 weight % CaO and 9 weight % P₂O₅.

In an alternative embodiment the bioactive glass composition may beprepared by the Melt method such as that described in U.S. Pat. No.5,981,412. Such a glass may have a composition of from 40 to 51 weight %SiO₂, 23 to 25 weight % CaO, 23 to 25 weight % Na₂O and 0 to 6 weight %P₂O₅. Preferably such a bioactive glass has the composition (by weight);

SiO2-45% NaO2-24.5% CaO-24.5% P2O5-6%.

Such a bioactive glass is available commercially as Bioglass® 45S5.

The manufacturing and processing methods used in the silicon basedbioactive glass family are ideally suited to the production of tailoredparticles for cutting under differing clinical conditions in restorativedentistry.

As mentioned above, hardening and softening components may be added tomodulate the hardness of the bioactive glass and hence control thenature of the substrate it is able to cut. Typically, alumina particlesare used in air abrasion systems. As can be seen from Table 1 aluminahas a Vickers Hardness of 2300, harder than both tooth enamel anddentine. Thus when using alumina as the cutting agent the operator mustcarefully control the extent of cutting so as not to damage the tooth. Abioactive glass having a Vickers Hardness greater than that of enamelwill cut through enamel and a bioactive glass having a Vickers Hardnessintermediate between enamel and dentine will cut through the latteronly. Thus, either by selecting from known bioactive glasses or byvarying the amounts of hardening agents the skilled man will be able toprepare bioactive glass air abrasive agents capable of cutting throughtooth enamel or dentine or both as necessary.

TABLE 1 Vicker's Hardness Numbers. Alumina 2000-2300 Glass beads 500-550Crushed glass powder 500-550 Polycarbonate resin 40-50 Demineraliseddentine model not recordable Enamel 300 Dentine (sound) 70 Bioglass ®45S5 458 +/− 9.4 Appatite/Wollastonite bioactive glass 680 58S Sol-gelbioactive glass (fully densified) 110

Cutting through enamel to gain access to decayed tooth substance ideallyrequires a hard bioactive glass such as Appatite/Woolastoniteglass-ceramics or the hard angular particles of crushed 45S5 Bioglass®.However, for selective removal of softened decayed dentine or thetreatment of exposed sensitive dentine surfaces a weaker and morerounded particle is desirable. By controlling the processing conditionsin the densification phase of the sol gel process (Hench L L., West JK., 1990, The Sol-gel Process, Chem. Reviews, 90, 33-72. Hench L L.,West J K., 1996, Biological applications of Bioactive glasses, LifeChemistry Reports, 13, 187-241.) sol-gel variants of bioactive glass canbe processed to differing densities and ultimate strengths andhardnesses to match resection or surface treatment needs. As shown inTable 1, a well densified 58S sol-gel Bioglass specimen yielded aVickers Hardness of approximately 110 (less densified specimens havelower hardnesses) compared with alumina 2,300, sound human enamel 300,sound human dentine 70, whereas decayed dentine is too soft to record.Thus, for selective removal of decayed dentine from a cavity, or forsclerosing or obliterating dentine tubules on an exposed externalsensitive dentine surface, to reduce or eliminate dentine sensitivity orpulpal pain, while minimising the damage to sound dentine, sol-gelbioglasses have the most promising physical characteristics.

Thus, by increasing the quantity of network modifier (non-silica speciesspecies, eg Na, K, Ca, Mn, Br, Al, N, P, Fl etc) the hardness of thefinished glass decreases. (see FIG. 8 a). These modifiers may be addedto the melt derived glasses while in their molten states, or to sol-gelmaterials at the mixing phase of production Hardness may also bedecreased by increasing the porosity within the glass, achieved byvariations in the drying and stabilisation and densification phases ofthe sol-gel process. As described above, the hardness of glasses can beincreased by allowing crystal formation within them, so the use of TiO2can act as a hardening agent, as the glass becomes a glass ceramic. Alsomodifications to the sol-gel processing phases allowing a more denseglass product will result in a harder product (see FIG. 8 b).

A further consideration when preparing a bioactive glass for use in thepresent invention is the shape of the bioactive glass particles. Thesemay be selected depending on the intended clinical application. Angularparticles are better suited to cutting through hard materials such asenamel whereas rounded particles are more suited to the removal of softtissue such as decayed dentine or sclerosing tubules on an exposedsensitive dentine surface. The shape of bioactive glass particles may becontrolled by selecting the appropriate particulation process from, forexample, grinding, crushing or air-collision milling during theirmanufacture. Thus, crushing produces sharper angulated particles,whereas, air collision milling will produce more rounded particles.Grinding (e.g. ball milling) however, will produce particles of a moreintermediate shape. These processes being suitable for glasses producedby both the sol-gel and melt routes.

Particles most suitable for use in the present invention will have adiameter in the range of 1 μm to 1 mm, more preferably in the range of10 μm to 500 μm.

Thus in treating a cavity one or more glasses may be employed to cutthrough the tooth enamel and/or dentine as required. Conventional airabrasion systems such as the Velopex® Alycat marketed by MedivanceInstruments Ltd. permit switching the source of the abrasive agent. Forcutting enamel the bioactive glass will preferably have a VickersHardness of at least 300 and the particle shape will preferably beangular. For selectively cutting dentine the glass will preferably havea Vickers Hardness of between about 70 and about 300 and the particleshape will preferably be more rounded. For selective removal of decayeddentine from a cavity, or for sclerosing or obliterating dentine tubuleson an exposed external sensitive dentine surface the glass willpreferably have a Vickers Hardness of between about 35 and about 150 andthe particle shape will preferably be rounded.

It is to be understood that the present invention covers allcombinations of suitable and preferred groups described hereinabove.

The present invention will now be illustrated, but is not intended to belimited, by means of the following examples.

EXAMPLE 1

To assess whether 45S5 bioactive glass will cut into and allow resectionof carious dentine when used as an abrasive powder in an air abrasionsystem.

Method

Five freshly extracted, retained human roots were collected from twoconsenting patients, according to the local Hospital Ethical CommitteeGuidelines. The criteria for acceptance were, that the roots should beintact after removal and have been diagnosed as having active ongoingcarious destruction across the entire root face, at the time oftreatment.

The teeth were washed in normal saline and transferred directly to thelab in moist conditions using sealed glass specimen containers. With theminimum of delay (so avoiding desiccation artefacts) the apical dentalrents were mounted on a solid metal baseplate, using a low temperaturethermoplastic “Dental Impression Compound” medium (Kerr Italia S.p.a,Salerno, Italy), with the carious root face uppermost.

Using a stainless steel traditional razor blade as a shield, anestimated 50% of each carious root face was protected, while the exposedarea was subjected to air abrasive cutting. 20-90 μm diameter 45S5bioactive glass particles were used as the abrasive, delivered through amodern commercially available “twin chambered” air abrasion machine(Medivance Instruments Ltd, London, England.). The abrasive wasdelivered through a 0.6 mm internal diameter nozzle at a constant 5 mmdistance from the target, over a 5 second period, using an accelerationpressure of 0.5 Mia and a medium abrasive powder flow rate (0.01 g persecond). All air abrasion activities were conducted within a purposebuilt self-evacuating chamber, to minimise environmental pollution(Handler, Westfield, N.J., USA). The five treated root faces weredesiccated using a conventional silica gel vacuum chamber set up, priorto carbon coating and scanning electron microscopic (SEM) examination.

Results.

FIG. 1 shows a representative image of the findings, clearly showing thecutting action that 45S5 bioactive glass has on carious root dentine,leading to removal of surface tissue, accompanied by smoothing &rounding of the treated surface, compared with untreated cariousdentine. Despite the short exposure time, significant decayed tissue wasremoved and the residual dentine surface showed characteristics of anair abraded surface.

The results clearly showed that 45S5 bioactive glass could removesoftened decayed dentine from a root surface when used as an airabrasive.

EXAMPLE 2

To establish whether the melt derived bioactive glasses would cut soundenamel and dentine and to examine any influence of the differentialhardness of the two substrates on the overall rate of substrate removal.Furthermore, to establish whether bioactive glass particles andfragments thereof were present on the residual cut surface and whetherdentine tubule orifices were closed or left patent at the surface aftertreatment.

Method

Five freshly extracted, human wisdom teeth were collected from fourconsenting patients, according to the local Ethical CommitteeGuidelines. The criteria for acceptance were, that the teeth should beintact after removal and have no clinical evidence of cariousdestruction or developmental anomaly, at the time of surgical treatment.

The teeth were washed in normal saline and transferred directly to thelab in moist conditions using sealed glass specimen containers. With theminimum of delay, (so avoiding desiccation artefacts) the teeth hadtheir pulp tissue removed and were sectioned axially, using awatercooled rotary diamond saw (Labcut 1010, Agar Scientific, Stanstead,Essex UK). The cut faces were then polished by hand to P1200 grit andacid etched in 37% phosphoric acid for 40 seconds (previously shown toremove all traces of a significant surface contaminant of silicon fromthe polishing process—identified in the SEM (Scanning ElectronMicroscope) using EDXA (Energy Dispersive X-Ray Analysis). The fivehemisected teeth thus yielded 10 specimens, which were serially mountedwith their sectioned surfaces uppermost and horizontal, on a solid metalbaseplate, using a low temperature thermoplastic “Dental ImpressionCompound” medium (Kerr Italia S.p.a, Salerno, Italy).

The prepared enamel/dentine surfaces were evenly subjected to airabrasive cuttings peening for a total of 30 seconds, during which time,the operator was required to treat the entire sectioned surface of thetooth evenly. 20-90 μm diameter 45S5 bioactive glass particles were usedas the abrasive, delivered through a modern commercially available “twinchambered” air abrasion machine (Medivance Instruments Ltd, London,England). The abrasive was delivered through a 0.6 mm internal diameternozzle at a constant 5 mm distance from the target, using anacceleration pressure of 0.5 MPa and a medium abrasive powder flow rate(0.01 g per second). All air abrasion activities were conducted within apurpose built self-evacuating chamber, to minimise environmentalpollution (Handler, Westfield, N.J., USA). The ten treated root faceswere desiccated using a conventional silica gel lab vacuum chamber setup, prior to carbon coating and SEM examination.

Results.

Five of the treated specimens were examined whole in the SEM (FIG. 2),and all showed a similar scalloped residual cut surface pattern overboth the enamel and dentine. Each showed a marked step height (20-30 μm)at the position of the Enamel-Dentine junction, the naturally softerdentine element having been removed to a greater extent than the harderenamel.

On examining the enamel surface structure itself, further evidence ofdifferential cutting was identified, as the Hunter-Schreger bands (awell recognised, normal anatomical structure) were readily identifiable,themselves being somewhat more resilient to air abrasive cutting (Boyde1984).

The remaining five specimens were cleaved using a Dental surgeon'sosteotome and mallet, first lodging the tooth firmly in the corner of apiece of heavy angle iron. The impact was targeted at the lowermostextremity of the tooth with the intention of cleaving the hemisectedspecimen axially, so revealing the untreated dentine tubule structure ina plane perpendicular to the exposed surface. FIG. 3 clearly showsdentine tubules coursing towards the treated surface, but none ends inan open orifice as one would expect to see if a phosphoric acid etcheddentine surface was similarly examined.

Furthermore, the cut/peened dentine surface illustrated in FIG. 3clearly showed evidence of residual particles on and embedded in thetreated surface. (It should be remembered that these particles hadwithstood the high energy cleaving impact, prior to SEM scanning.) EDXAanalysis of these particles revealed a clear silicon signal, indicatingit was debris from bioactive glass, as the polishing siliconadulteration was removed by the acid etch process described. This wasfurther confirmed by EDXA traces taken of material between the particlesfailing to register the presence of significant silicon peaks.

That the differential cutting was identifiable within one structure aswell as between two different elements of the tooth indicates that thehardness of the substrate will influence the rate at which it is cut bybioactive glasses. Thus, by matching the hardness of a bioactive glassto that of softened dentine will allow selective removal of diseaseddentine. Furthermore, materials matched to the hardness of intactdentine should peen or minimally cut the sensitive surface, whileoccluding its tubules, providing long-term pain relief. Such a materialshould have a negligible effect on the far harder surface enamel, whileremoving adherent tartar and unsightly staining deposits, thuscapitalizing on the differential cutting phenomenon.

The clear demonstration of residual bioactive glass particles andfragments (far smaller than the original abrasive employed, indicating ashattering of the abrasive on impact) resiliently sited on/in thetreated surface provides the vehicle for the desired bioactive responseof generation of a new calcium phosphate surface over the exposedtreated surfaces. By definition, the bioactive glasses all generate acalcium phosphate surface, overlying an ion depleted silica gel layer.This new physic-chemically created mineral surface (generated withoutcellular assistance or control) will allow the re-mineralisation andrepair of decayed tooth structures at the finished cavity surface—i.e.the limit of caries disease resection within a tooth, or allow atreated, sensitive exposed dentine surface to acquire a more resilientand long lasting desensitised mineralised surface. The hydration sourcewill be either super-saturated (Ca—P) solution of saliva, or the tissuefluid naturally found within the tubules of dentine, so rendering it a“wet” material in vivo. Both fluids are well recognised as abundant Caand P sources.

EXAMPLE 3

To establish whether the cut/peened surface created by the bioactiveglasses and having a deposition of bioactive particles and fragmentsthereof on the surface would withstand the rigours of an intra oralexistence and to identify any possible evidence of new calcium phosphatedeposits accreting on the cut surface.

Methods

Four volunteers agreed to have an intraoral prosthesis made, along thelines of a crib retained orthodontic appliance (inactive)-FIG. 4. Eachappliance bore four specimens of enamel and dentine that had beenpreviously rendered sterile against bacterial, viral and prion transfer,using two cycles of SDS detergent treatment (5% solution of Sodiumdodecyl sulphate for 24 hrs) (Azzopardi 2000: Measurement of erosion andprotecting exposed dentine with an adhesive resin coating:—an in vitroand in situ evaluation. PhD Thesis of GKT Dental Institute—Guy's Campus,King's College, University of London) and gamma irradiation (600 curiesat 22,272 Rad/hr Caesium 137 source Gammacell 1000 Elite Nordion Int.Inc. Ontario). A single 24 hour dose of Gamma radiation was used as itproved safest in a pilot investigation to test the method ofsterilisation (Azzopardi 2000).

Following the local Hospital Ethical Committee protocols, each of thedental test specimens were cut from a pair of undamaged extracted humanthird molars (which had previously had the pulp tissue removed), using awater cooled rotary diamond saw (Labcut 1010, Agar Scientific,Stanstead, Essex LTK). The slabs (16 in total) were polished to P1200grit and mounted in the acrylic baseplate using a cold cure orthodonticPMMA resin (Ortho-resin). The specimens were subsequently exposed to 37%phosphoric acid for 40 seconds to clean off their adulterated surfaceand to reveal the truly porous dentine structure (see example 2). Thecentral portion of each specimen was then protected using a 3 mm widestrip of PTFE tape, to avoid any contamination of this control site.

Using a stainless steel traditional razor blade as a shield, theanterior portion of each specimen only, was subjected to air abrasivecutting 20-90 μm diameter 45S5 bioactive glass particles were used asthe abrasive, delivered through a modern commercially available “twinchambered” air abrasion machine (Medivance Instruments Ltd, London,England.). The abrasive was delivered through a 0.6 mm internal diameternozzle at a constant 5 mm distance from the target, over a 5 secondperiod, using an acceleration pressure of 0.5 MPa and a medium abrasivepowder flow rate (0.01 g per second). By reversing the razor bladeshield again, the anterior ⅔ portion of each specimen was then protectedwhile the posterior portion underwent air abrasion with comparablediameter alumina particles. All air abrasion activities were conductedwithin a purpose built self evacuating chamber, to minimiseenvironmental pollution (Handler, Westfield, N.J., USA).

The air abrader instrument settings remained unchanged throughout theexperiment, although the lines were cleared of residual bioactive powderby allowing a 2 minute period of waste spraying into the “dust chamber.”The specimens were blown clean, using dry compressed air and theprotective tape was removed. The appliances were kept moist inorthodontic retainer boxes while awaiting periods of wear.

Following a well accepted daily wear protocol (Azzopardi 2000) theappliances were worn for eight hours per working day by all volunteers,but were removed at mealtimes in an attempt to allow hygienic handlingof the specimens at reviews (pre wear, at 3 days and 1 week). Eachreview, comprised examination of all three areas of each dentine/enamelspecimen with a tandem scanning confocal reflected light microscope(Noran Instruments Middlenton, Wis., USA) using a ×40/0.55 na dry lens(Nikon Corp. Japan), so avoiding any surface contamination withmicroscopist's lens oil. Digital images of representative portions ofeach surface were captured using an eyepiece mounted Coolpix 990 DigitalCamera (Nikon Corp. Japan). The mounted specimens could not berepeatedly examined in the SEM and Direct reflection imaging waspreferred to resin copying techniques as this avoided any furtherdisruption to the surface than was required.

Results.

FIG. 5 shows a montage of the images retrieved from one representativespecimen over the full week wear period. Images A and D correspond toarea (B) of FIG. 4 at the beginning and end of the trial period, imagesE and E correspond to area (D) of FIG. 4 at the beginning and end of thetrial period and images C and F correspond to area (A) of FIG. 4 at thebeginning and end of the trial period. The first apparent feature is thealteration of the open tubule dentine pattern by air abrasion/peeningwith both alumina and bioactive glass particles. An optically similarpattern is achieved, with closure of the open tubule orifices. In FIG.5, image D shows no degradation of the surface after a week's intra oralwear. The presence of blue on green in the image is an opticalphenomenon:—chromatic aberration, indicating a higher area of thesurface, which suggests the development of a new surface feature (Watson1997). The untouched dentine surface (E) showed a little similar changeover the same period, yet the alumina treated surface (C-before &F-after the trial period) showed none. The lack of similar additionalfeatures on the alumina surfaces implies a resilient accretion ormineral growth phenomenon, accelerated on the dentine surfaces treatedwith bioactive glasses. If the phenomena was transient debris, it stoodan equal chance of appearing an the other surfaces too, yet none wasfound in any of the specimens examined.

That similar new accretions were not seen on the alumina treatedsurfaces also fits with the suggestion that this new maternal is indeeda new calcium phosphate deposit, as in the presence of greater than 3%alumina, the bioactive reaction is known to be killed (Hench 1998).

By way of confirmation of the previous results, it was noted thatwithout exposing the teeth to the desiccation necessary for SEMpreparation, the classical step height was maintained in the bioactiveglass sprayed EDJ regions, indicating that the dentine was truly removedat a faster rate than the harder enamel. The alumina sprayed surfaceshowed a more rapid removal of tissue (FIG. 4) and the EDJ step heightwas far less prominent as both substrates were so much softer thanalumina particles.

The images shown clearly demonstrate the altered surface achieved usingbioactive glasses as air abrasives. The maintenance of closure of thetubule orifices during intra oral wear suggests the surface is resilientand the altered morphology over time, further substantiates the claimthat the bioactive glass abrasive debris is capable of seeding thegeneration of a calcium phosphate mineral surface, in the intra-oral inservice environment, at a rate far faster than exposed dentine and thattreated with alumina.

EXAMPLE 4

Corroborative demonstration of the differential cutting of similar sizedparticles of different hardness.

Methods

Two monolithic slabs 1 cm×1 cm×3 mm deep of 58S sol-gel bioactive glasswere sawn into comb shapes using a diamond wire saw (Bennettech,Leicester, England). Each tyne of the comb was 2 mm wide and 6 mm long.Resting each comb on a bed of low temperature thermoplastic “DentalImpression Compound” medium (Kerr Italia S.p.a, Salerno, Italy), allowedthe test substrate to be held in a horizontal position in the sprayingchamber. Using stainless steel razor blades as protective shieldsbetween the tynes, each test surface could be air abraded withoutdamaging or adulterating the neighbouring specimens. Each comb yieldedthree tynes, affording three each of two abrasive test sites.

Each specimen was subjected to air abrasive cutting, using either 20-90μm diameter 45S5 bioactive glass particles or a similarly sized aluminaparticulate, delivered through a modern commercially available “twinchambered” air abrasion machine (Medivance Instruments Ltd, London,England.). The abrasive was delivered through a 0.6 mm internal diameternozzle at a constant 5 ml distance from the target, over a 5 secondperiod only, using an acceleration pressure of 0.5 MPa and a mediumabrasive powder flow rate (0.01 g per second). All air abrasionactivities were conducted within a purpose built self evacuatingchamber, to minimise environmental pollution (Handler, Westfield, N.J.,USA). The air abrader instrument settings remained unchanged throughoutthe experiment, although the lines were cleared of residual bioactivepowder by allowing a 2 minute period of waste spraying into the “dustchamber.” The specimens were blown clean, using dry compressed air andthen transferred to the SEM facility for gold coating and imaging.

Results.

FIG. 6 is presented as representative of the findings. The surfacestreated with 45S5 bioactive glass were indented far less than thosesubjected to alumina treatment.

The 45S5 bioactive glass left residual particles of itself imbeddedin/on the cut surface, whereas the harder grit produced a cleanersurface. (A plug of material seen impacted in the base of the aluminaresection area was due to choking of the cutting apparatus.)

The data shown in FIG. 6 clearly demonstrates that abrasive aggregatesof differing hardness will have very different cutting effects whenapplied to the same substrate under the same cutting conditions, sosupporting the proposition of differing abrasive properties havingdiffering cutting, finishing and minimally resecting surface treatmentroles.

EXAMPLE 5

Demonstration of tubule closure after use of the invention in an in-vivomodel.

Methods.

The experiment established in Example 3 was allowed to continue for aperiod of 13 days in total, at the end of which, the specimens wereretrieved from the base-plates by careful sectioning, ensuring there wasno contamination of the exposed, treated dentine surfaces. Each specimenwas clearly marked to allow accurate re-orientation. Following a wellestablished practice for identifying the movement of dentine bondingagents through tooth tissue (Griffiths B M, Watson T F, Sherriff M. 1999The influence of dentine bonding systems and their handlingcharacteristics on the morphology and micro-permeability of thedentine-adhesive interface. J. Dent. 27 63-71), an excess of Rhodamine-Blabelled dentine bonding resin (EBS Bond Espe, Seefeld, Germany) waspuddled over the horizontally orientated dentine test surfaces andallowed to soak into the tubules as best it could over a two hourperiod. The resin's polymerisation and set, was avoided by keeping thesamples in total darkness for the experimental period. (Old photographicfilm containers served well as light proof chambers.) No sample showedpremature set of the resin at the end of the soak phase. The Resin wasconventionally command set at the end of the two hour period using themanufacturer's supplied 470 nm wavelength curing light.

Using a water-cooled rotary diamond saw (Labcut 1010, Agar Scientific,Stanstead, Essex UK), the specimens were sectioned to reveal anylabelled resin penetration perpendicular to the experimental surface(see FIG. 7.) the cut faces were then polished by hand to P1200 grit,prior to viewing with a tandem scanning confocal fluorescence lightmicroscope (Noran Instruments Middlenton, Wis., USA) using a ×60/1.40 naoil lens (Nikon Corp. Japan) and ×10 objective giving a magnification of×600 for the images (FIG. 7.) The confocal microscope allowed subsurface imaging in such translucent specimens, so avoiding confoundingby any smear layer from the cutting/polishing phase.

Results.

FIG. 7. Composite image showing tandem scanning confocal fluorescenceimaging of a representative dentine tooth slab, hemisected after 13 daysintra oral wear. The specimen is shown, opposite Part A, the Bioactiveglass treated area, and no labelled resin penetration is seen.Occasional shadow marks of empty tubules are seen, illuminated by thefluorescence signal from the surface resin. Note that no tubule markingsare visible at the tooth-resin interface, indicating the development ofa new sealing surface layer. Part B shows the response of exposeddentine to the same intra-oral environment. Some patent tubules remain,although most are sealed off within their lumens, indicating adifferent, slower, stenotic mechanism. Part C shows the alumina treatedsurface of the same specimen. Clearly, the tubules have not been closedand no additional remineralisation is able to occur, presumably becausethe bioactive response is being poisoned by the alumina debris. Thus itwould seem that the currently available abrasive, while cutting rapidly(note excessive loss of tissue height in the alumina treated region-C ofspecimen view.) hinders all chance of any surface reminerlisation, bywhatever mechanism. Furthermore, the bioactive glasses accelerate theformation of a new mineralised surface, providing a seal faster than theuntreated dentine surface.

Discussion

As is clearly seen from the images in FIG. 7, that over the experimentaltime period, the dentine was able to partially heal itself, as only fewresin tags were present in the sectioned faces examined. This was anexpected finding as it is known that dentine can slowly sclerose anyopen tubules by CaP crystal growth, when in a suitable environment. Thedentine surfaces treated with bioactive glasses however, did not permitany access to any of the labelled resin, only the shadows of the emptyclosed off tubules could be seen, illuminated by the radiant light fromthe labelled resin on the dentine surface. The images also show that fewif any tubules extend to the tooth-resin interface, implying aperipheral closure of the tubules by new mineral deposition, unlike theexposed dentine interface, where resin can be seen entering open tubuleorifices. The sclerosis/stenosis process would seem to be occurringwithin the patent tubule, discriminating between this process and thesurface healing phenomenon seen in the bioactive glass treated surfaces.

The dentine treated with alumina showed a remarkable degree of leakage,the image in FIG. 7, clearly showing labelled resin permeatingthroughout the tubule network in the imaged field. The presence ofgreater than 3% alumina in the vicinity of a bioactive process is knownto kill the reaction completely. It would seem that this has occurred inthis experiment, the treated surface being rendered incapable of furtherCaP salt crystallisation. This evidence confirms that the deposition ofbioactive glasses on tooth surfaces cut or peened by this method, canallow accelerated mineral crystallisation on the treated areas, alteringthe surfaces in a beneficial way for use in the resection, arrest andtreatment of dental caries, dentine hypersensitivity and pulpal pain,congenital dental hard tissue defects, discolouration and tooth wear.

1. A method of polishing at least one tooth structure of a personcomprising applying bioactive glass particles directly to said at leastone tooth structure of said person using an air abrasion system.
 2. Amethod of cleaning at least one tooth structure of a person comprisingapplying bioactive glass particles directly to said at least one toothstructure of said person using an air abrasion system.
 3. A methodaccording to claim 1, wherein the bioactive glass comprises a source ofSiO₂ or Si(OH)₂, and a source of CaO₂ and/or P₂O₅.
 4. A method accordingto claim 3, wherein the bioactive glass further comprises at least onehardening agent and/or at least one softening agent.
 5. A methodaccording to claim 4, wherein the softening agent is selected from Na,K, Ca, Mg, B, Al, P, N, F and the hardening agent is TiO₂.
 6. A methodaccording to claim 1, wherein the bioactive glass comprises 1 to 100%SiO₂ or Si(OH)₂, 0 to 60% CaO, 0 to 60% P₂O₅, 0 to 45% Na₂O, 0 to 45%K₂O and 0 to 40% MgO.
 7. A method according to claim 6, wherein thebioactive glass comprises 44 to 86 weight % SiO₂, 4 to 46 weight % CaOand 3 to 15 weight % P₂O₅.
 8. A method according to claim 7, wherein thebioactive glass comprises 58 weight % SiO₂, 33 weight % CaO and 9 weight% P₂O₅.
 9. A method according to claim 6, wherein the bioactive glasscomprises 40 to 51 weight % SiO₂, 23 to 25 weight % CaO, 23 to 25 weight% Na₂O and 0 to 6 weight % P₂O₅.
 10. A method according to claim 9,wherein the bioactive glass comprises (by weight): SiO₂-45% CaO-24.5%Na₂O-24.5% P₂O₅-6%.
 11. A method according to claim 1, wherein thebioactive glass is obtainable by the Sol-Gel method.
 12. A methodaccording to claim 1, wherein the bioactive glass is obtainable by theMelt method.
 13. A method according to claim 1, wherein the bioactiveglass comprises 45S5 (Bioglass®).
 14. A method according to claim 1,wherein the bioactive glass has a Vickers Hardness of at least that ofhealthy tooth dentine and at most of that of tooth enamel.
 15. A methodaccording to claim 13, wherein the bioactive glass has a Vickershardness of at least about 70 and at most about
 300. 16. A methodaccording to claim 1, wherein the bioactive glass particles aresubstantially spherical.
 17. A method according to claim 1, wherein thebioactive glass particles have a diameter of from 10 μm to 500 μm.